The present invention relates generally to specimen mounts for use with transmission electron microscopes, and more particularly to a power probe for in-situ viewing of electromigration in integrated circuits and in-situ operation of integrated circuits or microprocessors.
Electromigration in aluminum thin film conductors is a likely source of unreliability in integrated circuits. Electromigration is the mass transport of atoms in a conductor under a current stress. The interconnections inside integrated circuits, called lines, stripes or linestripes, are generally made of a thin aluminum or aluminum alloy (Al-1% Si) film. Electromigration in aluminum can occur, at elevated temperatures (&lt;600.degree. C.), at current densities greater than 10.sup.5 amps/cm.sup.2. With the maturing of Very Large Scale Integrated (VLSI) technology, decreasing feature size and increasing gate densities (exceeding 10,000 gates/cm.sup.2) are resulting in higher current densities exceeding 10.sup.6 amps/cm.sup.2, while linestripe widths are reaching into the submicron range.
The failure modes in aluminum thin film conductors from electromigration are characteristically cracks, voids or hillocks in the linestripes, resulting in open circuits. Most of the research to date of electromigration in aluminum conductors has been directed to examining the formation of these voids and hillocks. However, experiments have also shown that short-circuits caused by whisker formation between adjacent stripes, or between multi-level structures, can be equally damaging to integrated circuit performance. Because of the importance of this failure mode in integrated circuits, and so that means for reducing its effect may be developed, further research in the nature of this phenomenon is clearly needed.
Descriptions of previous investigations for gaining improved understanding of electromigration in aluminum films in-situ in transmission electron microscopes (TEMs), in which investigations the inventors have variously participated, may be found in "Crystallographic Orientation of Aluminum Whiskers Formed by Electromigration Using Transmission Electron Microscopy," D. R. Kitchen, S. L. Linder, R. E. Omlor and P. F. Lloyd, Proceedings of the 45th Annual Meeting of the Electron Microscope Society of America, 1987; "Sample Preparation of Aluminum Bridge Test Vehicles for TEM In-Situ Crystallographic Studies of Electromigration," W. E. Rhoden, J. V. Maskowitz, D. R. Kitchen, R. E. Omlor and P. F. Lloyd, Material Research Society Symposium Proceedings, Vol. 115, 1988; "Mark II Test Vehicle Holder for In Situ Viewing of Integrated Circuit Electromigration, " J. V. Maskowitz, W. E. Rhoden, D. R. Kitchen, R. E. Omlor and P. F. Lloyd, Proceedings of the 44th Annual Meeting of the Electron Microscopy Society of America, 1986; and, Electron Microscopy Observation of Electrotransport. J. V. Maskowitz and W. E. Rhoden, MS Thesis, GE85D-77, School of Engineering, Air Force Institute of Technology, Air Force Technical Report No. AFIT/GE/ENG/85D-25, December, 1985.
To perform these and similar studies, an aluminum linestripe has to be freely suspended, without an interfering silicon substrate, inside the ultra-high vacuum and in the electron beam path of a transmission electron microscope. Electrical current has to be supplied to the linestripe inside the TEM so that electromigration can be viewed in real time. As described in the studies, to achieve this the investigators first constructed an unique Bridge Test Vehicle (BTV) in which substrate material was etched away from vapor-deposited aluminum film to leave freely suspended ("bridge") test patterns of aluminum linestripes supported by the surrounding unetched test vehicle structure for mounting on a holder. A JOEL EM-SHH heating holder, manufactured by JOEL USA, Inc., Boston, Mass., was modified to hold the Bridge Test Vehicle in the path of the electron beam. A heating holder provides a means for heating and holding a specimen in the electron beam path. It mounts through a side hole, or goniometer, in an electron microscope and includes vacuum sealing at the side hole and electrical leads through the holder to power the heater at its tip.
The modified holder, called a Mark II holder, provided the technical means to prove the concept of powered circuit operation inside the ultra-high vacuum environment of an transmission electron microscope. Before its invention, similar electromigration studies could be performed only by first applying a current to the test sample, or specimen, in the open air and then placing the test sample inside the TEM for investigation. After viewing, the sample was removed and the current reapplied, followed by again placing inside the TEM. This cycle was repeated with successively higher current densities until the test was completed or the sample failed. This testing method particularly suffered from the contaminating effect of the atmosphere and handling while under test. Also, artificial electric, thermal and mechanical stresses were placed on the sample from starting, stopping, and restarting the current. Additionally, structural changes occurring in the sample could not be viewed or recorded in real time as they occurred. The Mark II holder for the first time permitted investigators to perform real time electromigration studies inside the ultra-high vacuum environment of a transmission electron microscope. The Mark II holder validated the concept of powered electronic devices operating inside the ultra-high vacuum environment of a transmission electron microscope.
Despite its substantial contribution to the prior art, the Mark II holder nevertheless had several deficiencies. For example, changing specimens was particularly time-consuming and inconvenient. At least one day was required to remove and replace a test specimen. Also, the small specimen area limited the size of the test vehicle. Wire bonding to the test vehicle was difficult due to the awkward orientation of the test vehicle to the holder. The brittle nature of the power leads and limited access to them added to the difficulty of making electrical connections to the test vehicle. Due to the nature of its adaptation from the JEOL heating holder, only two linestripes could be tested in-situ at ultra-high vacuum. The power application and distribution to the linestripe or test object was severely limited. Also, the temperature of the linestripe (or thin film) could not be regulated to any extent. Finally, only linestripes could be exercised and not integrated circuits or microprocessor devices.
Thus it is seen that there is a need for an improved specimen or test vehicle holder that includes all the advantages of the Mark II holder without its deficiencies.
It is, therefore, a principal object of the present invention to provide an improved test vehicle holder for use inside a transmission electron microscope that permits real time electromigration studies to be performed inside a high-vacuum environment such as the interior of an electro microscope.
It is another object of the present invention to provide an improved test vehicle holder that permits rapid and convenient changing of test samples and specimens.
It is a further object of the present invention to provide an improved test vehicle holder that permits multiple exercising of many different linestripes.
It is yet another object of the present invention to provide a test vehicle holder that permits adequate temperature compensation during an experimental run.
It is yet a further object of the present invention to provide a test vehicle holder that permits VLSI/VHSIC circuits and microprocessor devices to be examined inside a transmission electron microscope while specific locations, such as linestripes, memory cells, nodes, logic gates, active transistor areas, capacitors and so forth, are exercised.
It is still another object of the present invention to provide a test vehicle holder that permits convenient interface with commercial electronic test signal generators, monitors and analysis devices.
It is a unique feature of the present invention that it will permit reliability testing of integrated circuits with known faults inside a transmission electron microscope.
It is another feature of the present invention that it will permit the study of electron beam induced current effects inside an integrated circuit.
It is a further feature of the present invention that it will permit examination of integrated circuits by voltage contrasting.
It is yet another feature of the present invention that it will permit morphology changes of polycrystalline linestripes to be predicted.
It is yet a further feature of the present invention that it will also work inside other viewing apparatus such as a scanning electron microscope, a microprobe, a scanning Auger microprobe and an Electron Spectroscope for Chemical Analysis (ESCA).
It is a principal advantage of the present invention that it will permit real time exercising of seven test stripes or specimens for electromigration studies.
It is another prime advantage of the present invention that it will distribute power to multiple locations in an examination area while simultaneously maintaining temperature heating or cooling.
It is a further advantage of the present invention that its manufacture will be straightforward and its operation convenient.
It is yet another advantage of the present invention that it is compatible with test devices that generate continuous AC, DC or pulsed signals.
It is yet a further advantage of the present invention that it is compatible with straightforward commercial electronic performance monitors and analyzers.
It is also an advantage of the present invention that it will permit the exercise, observation and subsequent performance analysis of single or multiple circuit design features to include gates, vias, metallization lines, polycrystalline lines, bonding pads and wire interconnects.
It is still another advantage of the present invention that it will permit chemical analysis of integrated circuits when used with an Electroscope for Chemical Analysis (ESCA).
These and other objects, features and advantages of the present invention will become apparent as the description of certain representative embodiments proceeds.